The invention concerns a safety valve, in particular for low pressures and aggressive media, with an inlet which ends at the top in a valve seat upon which lies in a sealing manner a weight-loaded sealing body, whereby a certain overpressure below the valve seat and/or a certain underpressure above the valve seat causes the sealing body to lift.
Low-pressure safety valves are used for pressure safety against overpressure or vacuum in pressure systems, for example reservoirs, boilers, pipelines. These low pressures lie in the region of  less than 200 mbar. Since pressure safety with highly aggressive and environmentally critical media requires hermetic sealing of the valve to the atmosphere, such low-pressure valves are usually designed in such a way that the component, for example a spring or a weight, loading the sealing body, in particular the valve plug, is placed directly in the product-contacting space of the valve body. In this way, one avoids stem lead-throughs to the atmosphere which have to be sealed off with sealing elements such as bellows or a diaphragm. The materials resistant to highly aggressive media, for example Hastelloy, PTFE, etc., result in bellows/diaphragms whose operating force is so large due to spring stiffness, that this force acts counter to the effective valve opening or closing force in the low-pressure range and disturbs the valve function.
A weight-loaded low-pressure safety valve is known in which the valve plug has a cylindrical shaft guided in a bore arranged in the valve bonnet. Due to the necessary clearance and the relatively great distance of the lodging point from the sealing surface of the seat/plug shut-off element, an eccentricity of the valve plug axis relative to the valve seat axis develops. This poor centering leads to an irregular distribution of forces at the valve plug and hence to a tilting to one side during opening of the valve. This results in leakage even before the trip pressure is reached and a jamming of the guide due to tilting.
Another valve design employs a valve plug with guide webs which is inserted in the valve seat bore. This design has the disadvantage of narrowing the valve cross-section due to the guide webs and thus reducing the valve performance. In addition, the elevated center of gravity of the weight arranged above the valve plug causes the valve plug to tilt or jam during opening. The weight loading the valve plug is designed to match a defined valve trip pressure. Any subsequent change requires the weight to be replaced or reworked. The weight is manufactured either from metal or plastic. When used for highly aggressive media, metallic weights made from special materials, for example Hastelloy, tantalum, zirconium, etc., are required, which are correspondingly costly. If weights made of resistant plastics, for example PTFE, are used, the low density of the plastic relative to the metal leads to large-volume weights and hence to large-volume valve bodies.
The object of the invention is to improve a safety valve of the kind mentioned at the outset in such a way that it moves very smoothly in all operating conditions while providing a high degree of sealing. A further object of the invention is to ensure that the total weight loading the sealing body can be easily adjusted and varied.
These problems are solved according to the invention in that, at its upper end, the sealing body forms a shaft projecting vertically upward which is guided along or in a vertical part fixed immovably to the valve body and that two or more weights, particularly detachable weights, are fastened to or on top of the shaft.
The shaft of the sealing body is so reliably guided that a great smoothness of motion is achieved. The weight of the sealing body can also be very precisely determined by the number and the selected weight of the individual weights.
It is preferentially proposed that the shaft projecting upward comprises a vertical inner bore open at the top in which lies a rod fixed to the valve body. It is also advantageous when the rod is jacketed with a material resistant to aggressive media.
Simple manufacture and assembly are achieved when annular weights lie one above the other on the shaft, surrounding the shaft. It is also advantageous when the weights are jacketed with or consist of a material resistant to aggressive media.
A secure hold of the weights is achieved when a disc, particularly of a material resistant to aggressive media, lies above the weights, which is secured on the shaft by a round cord or a circlip.
Exact locking in place of the weights and especially the annular disc is achieved irrespective of the number of weights when the outer side of the shaft comprises several annular grooves spaced at distances corresponding to the weight heights.
The smoothness of motion of the sealing body is improved when the guide rod contacts the inner bore of the shaft via only two annular projections. It is furthermore advantageous when the inner recess of the shaft is connected with the valve interior space by at least one venting bore.
In order to ensure that the sealing body does not close immediately after opening it is proposed that the sealing body comprise on its underside a recess, in particular a concave, conical edge outside the valve seat. The sealing body is thus carried for a certain time by the gas or liquid emerging from the outlet.